Absorption in the Ultraviolet and Visible Regions of

March 20, 1957

SPECTR.4 O F cHLOROAQUOCIIROMIUM (111) IONS IN A C I D A f E D I A

sure of the gas samples was between 30 to 34 mm. this small amount would not affect the vapor pressure measurements. Acknowledgment.-The assistance of Mr. J. Baudian of the Perkin-Elmer Corp., and Mrs. Elsie

[CONTRIBUTION FROM THE

1281

Dupre and Mr. R. J. O’Connor of the Southern Utilization Research Branch, Agricultural Research Service, U.S.D.A., in obtaining the infrared spectra is gratefully acknowledged. NEWORLEANS18,

DEPARTMENT OF CHEMISTRY,

LA.

UNIVERSITY OF MICHIGAN]

Absorption in the Ultraviolet and Visible Regions of Chloroaquochromium(II1) Ions in Acid Media BY PHILIP J. ELVING AND BERNARD ZEMEL RECEIVED OCTOBER10, 1956 The absorption spectra of the hexaaquo-, chloropentaaquo- and dichlorotetraaquocliromium(III) ions were observed in the presence of high hydrogen ion concentration t o minimize the contribution of basic and polymeric chromium species. I n solutions of high perchloric acid concentration the positions of the band maxima for the three ions shifted toward the red in a regular manner with increase in the number of complexed chloride ions. In the presence of 12 M hydrochloric acid, the absorption spectra obtained fitted this pattern in a way consistent with the formation of a new species in solution: trichlorotriaquochromium( 111); the latter does not occur t o any extent in concentrations of hydrochloric acid much below 12 M. Since this species was not formed in saturated ( 5 M ) calcium chloride solution, its formation is apparently highly hydrogen-ion dependent; a mechanism for formation of the [Cr(H~0)&13]species is suggested. The structure of its spec+ to be the cis form. trum indicates [Cr(H20)aCl~]

The absorption bands of complexes of the transition metals in the visible and ultraviolet are generally of two kinds: those due to electron transfer processes between metal and ligand, and those arising from transitions within the d-shell of the ion.’ The relatively intense bands in the ultraviolet having molar absorptivities of l o 3or greater are considered to be electron transfer bands; bands having molar absorptivities below ca. lo2 are treated as transition spectra. It is these relatively weak transition bands which are of particular interest in the chromium(II1) complexes and which account for the characteristic colors of chromium(II1) solutions, e.g., the violet of the hexaaquo ion, the light green of the chloropentaaquo ion and the dark green of the dichlorotetraaquo ion. The chromium(II1) ion lies within a negative field due to its co6rdinated ligands. In the presence of this field, the degeneracy of the ground state is removed, and the d-orbital set is split into two multiplet levels. The two absorption bands in the visible correspond to the two allowed transitions between these levels. Following the isolation and identification of the chloroaquochromium(II1) complexes, several investigators2 reported absorption spectra for them ; this work was qualitative in nature and served merely to confirm the existence of absorption bands. Bjerrum3 in a very carefully performed set of experiments, reported molar absorptivity values for the wave length region from 450 to 700 mp; however, this region includes only one of the two transition bands. Later, Sueda4 published absorption curves for the 250 to 470 mp region; unfortunately, he had difficulty with impurities in solution, and some, if not all, of his results seem (1) E. Rabinowitch, Reus. Modern P h y s . , 14, 112 (1942); H. Hartmann and H. L. Schlafer, Angew. Chem., 66, 768 (1954). (2) A . B y k and H. Jaffe, Z. physik. Chem., 68, 323 (1909); H . C . Jones and W . mi. Strong, Physik. Z . , 10, 499 (1909); H. C. Jones and J. A. Anderson, “The Absorption Spectra of Solutions,” Carnegie Publication 89. Washington, 1923. (3) N. Bjerrum, Z. onorg. Chem , 63, 140 (1909). (4) H. Sueda, Bidi Chem. S O L .J a p a n , 12, 480, 524 (1937).

questionable. Since the hexaaquochromium(II1) ion is a starting material for the substituted ammino and thiocyanato complexes, its spectrum has been reported in connection with investigations on these system^.^ The objective of the present investigation was to examine and correlate the absorption spectra of the chloroaquochromium(II1) ions in aqueous solution under such circumstances that the state of the chromium(II1) species would be well-defined. Consequently. the absorption spectra of the hexaaquo-, the chloropentaaquo- and the dichlorotetraaquochromium(II1) ions were investigated in solutions of high perchloric acid and high hydrochloric acid concentration ; under such conditions, the contributions of basic and condensed chromium species are negligible, and the only ligands available in solution for complexing chromium are the chloride ion and the solvent water. Absorption Spectra of the Chloroaquochromium(II1) Complexes Perchloric Acid Solution.-Absorption spectra of [Cr(H20)6]+3 ion in perchloric acid solution in the region from 200 to 800 mp show the expected broad bands (Figs. 1 and 2 ) . In addition to maxima a t 407 mp (UM = 16.1) and 575 mp (UM = 13.9), there is a shoulder a t 670 mp and a steep cutoff in the far ultraviolet with a shoulder a t 260 mp. Above 350 mp, the spectrum is independent of perchloric acid concentration up to 6 M and follows Beer’s law for concentrations below 0.08 M in chromium. The spectrum in the far ultraviolet is a function of both perchloric acid and chromium concentrations. The absorption bands in the spectra of [Cr(H20)5CI]++ and [Cr(H2O)4Cl2]+ions, obtained under similar conditions, show the expected bathochromic effect (Figs. 1 and 2) ; the band shapes are, however, different. The [Cr(HzO)sC1]++ ion shows ( 5 ) (a) R . I. Coleman and F. W . Schwartz, THISJOURNAL, 54, 3206 (1932); (b) E . L. King and E. B . Dismukes, ibid , 7 4 , 1674 (1952); ( c ) 7 Bjerrum and A . Lamm Acta C h r m Scand , 9 , 216 (1958).

1282

PHILIP

Vol. 79

J. ELVIXG AND BERNARD ZEhZEL

1.5

d

;

e 3 1.0 c

,I_

-i

400

500 WAVELENGTH

600 MILLIMICRONS

700

Fig. 1.-Molar absorptivity of chloroaquochromiumj 111) ions in 6 M perchloric acid: ( A ) hexaaquochromium(II1) ion; ( B ) chloropentaaquochromiuxn(II1)ion; ( C ) dichlorotetraaquochromium(II1) ion; ( D ) absorbance of chromium(II1) in 12 M hydrochloric acid.

maxima a t 430 mp ( a =~21.6) and 605 nip ( U M = 17.8); the latter band is obviously complex and contains several components. The [Cr(H20),Clz]+ ion shows maxima a t 450 mp (ahf = 27.0) and 635 mp ( a =~ 23.9); the latter band has shoulders a t 650 and 690 mp, indicating its complex nature. Beer’s law is followed for concentrations below O.OG in the region above 330 mp; a t higher concentrations, the molar absorptivities are lower than expected, the short-wave band decreasing proportionately somewhat more. The far ultraviolet spectra for both ions are dependent on both acid and ion concentrations. Hydrochloric Acid Solution.-Although the conversion to lower, i.e., less chloride present, complexes is inhibited in acid solution, a slow transformation of the higher complexes involving loss of chloride goes on even in concentrated acid solutions. It is of interest to compare the spectra of the chromium complexes in acid media of high chloride concentration. Absorption spectra for solutions of hexaaquochromium(II1) perchlorate and of dichlorotetraaquochromium(II1) chloride in 12 M hydrochloric acid were measured over the range of 350 to SO0 mp immediately after mixing and a t intervals for several days. The resultant spectra showed a strong bathochromic effect (Fig. 1) similar to that observed in building up the lower chloro complexes. A shift in the spectra was noticeable within a few minutes after mixing ; the change was essentially complete within 3 to 4 hr. The final spectrum was independent of the nature of the starting chromic complex and showed no further change on standing. Dilution of the greenish-yellow solution with water caused a rapid degradation to the green dichloro species. The positions of the absorption band maxima in the spectra of the lower chromic chloride complexes are compared with those of the species

0.5

1

I

I

I

236

250 262 274 Millimicrons. Fig. 2.-Ultraviolet absorbance of chloroaquochromiuni (111) ions in 6 M perchloric acid: ( A ) hexaaquochromiuni (111) ion; ( B ) chloropentaaquochromium(II1) ion; ( C ) cliclilorotetraaquoclironiiuin(111) ion.

present in concentrated hydrochloric acid in Table I. The trichlorotriayuochroniium(II1) complex has been prepared6 as a brown solid; its solution is originally greenish yellow in color and rapidly turns to the dark green of the dichloro complex. No compound of the ion [Cr(1120)2C14]-has been reported; ions of the composition [Cr(H20)C16]= dBSORPTION

TABLE I MAXIMA FOR THE CHLOROAQUOCHROMIUM(III) COMPLEXES~ Position of band maxima,c mp I I1

Species

Bathochromic shift in maxima, mlr I I1

[Cr(H20)6] +++

407 575 (16.1) (13.8) [Cr(H20)6CI] 430 605 23 30 (21.6) (17.8) 20 30 lCr(HzO)4C121 4 50 635 (27.9) (23.0) 475 665 25 30 (in 12 x HCI)* Dea In 6 M perchloric acid media except as indicated. termined with a Cary Model l l spectrophotometer a t 23”. The molar concentrations of the Cr(II1) species in the perchloric acid experiments varied in different runs between 0.02 and 0.05 M , and in the hydrochloric acid experiments were uncertain, but between 0.01 and 0.04 M . *Species is considered to be [Cr(H*0)3C13]. The U M values are somewhat higher, perhaps by about 30y0,than those for the dichloro species in perchloric acid solution. e Values of the molar absorptivity, u H , are given in parentheses below the appropriate wave lengths. ++

+

( 6 ) C . Brauer, “ H d n d h u c h der Praparativen anorganischen Chemie.” Enke, Stuttgart, 19.54, p. 1032. ( 7 ) (a) A . Werner and A. Gubser, Ber.. 3 4 , 1579 (1901); ( b ) R . Aliegg and F, Auerbach, “HandLuch cler anorganischen Chemie.” Vol. 4 , Ilirzel, Leipzig, 1921, p . 100.

March 20, 1957

SPECTRA OF CHLOROAQUOCHROMIUM(III) IONSIN ACIDMEDIA

1283

and [CrCIB]--- 7b have been reported as red crystalline alkali metal salts, the transiently red solutions of which yield only the green dichloro species. Hydrochloride complexes, Cr(HCl),+++, have been reported a t -50°, but are unstable a t room temperature? Since the addition of concentrated perchloric acid involving a higher hydrogen ion activity than that in 12 M HC1 produces no spectral shift in the green dichloro complex, it seems highly improbable that hydrochloride complexes are involved in the present situation. From the foregoing discussion and from comparison with the spectra of the lower members of the series (Table I), it appears reasonable to assign to the complex species present in concentrated hydrochloric acid the formula [Cr(H20)3Cla]. Calcium Chloride Solution.-The polarographic half-wave potential observed for chromic chloride in saturated calcium chloride solution (cu. 5.6 M ) is over 0.1 v. more negative than that found in 1 M potassium chloride ~ o l u t i o n . ~ Since this type of behavior is often associated with complex formation, the spectrum of the [Cr(Hz0)4C12] ion in saturated calcium chloride solution was measured; i t was identical to that in perchloric acid; positions of the band maxima agreed within experimental error. The shift in half-wave potential is apparently associated in this case with phenomena other than the formation of a new complex. There remains, however, the possibility that the transition is merely very slow. Discussion of Spectrophotometric Data In perchloric acid solutions the absorptivities of the hexaaquo-, the chloropentaaquo- and the dichlorotetraaquochromium(II1) ions were measured for the region from 750 to 200 mp; the spectral profiles are different from those reported in the literature. Each spectrum consists of three bands: one band in the far ultraviolet, due to electronic transitions, extends beyond the ultraviolet cutoff of the spectrophotometers used, but has a shoulder in the region between 200 and 350 mp; one band in the region between 400 and 450 mp; and a complex band in the region between 575 and 650 mp. There is a regular shift of about 30 mp for the longwave band for each chloride ion complexed. The molar absorptivities for the dichloro species for the wave length region of 350 to 750 mp are given in Table 11; the solutions follow Beer’s law for concentrations below 0.05 M. I n the region below 300 mp all three species show the very steeply rising electron transfer band (Fig. 2) which acts as a cutoff in the ultraviolet; the maxima all lie below 200 mp. Both the hexaaquo and the chloropentaaquo electron transfer bands have shoulders, but the bands all change markedly with both perchloric acid and chromium(II1) concentration so that i t is difficult to analyze them. Although the absorption spectra of [Cr(H20)4Clz]+ ion follow Beer’s law for a given Cr(II1) concentration range, the spectral profile varies with

TABLE I1 MOLAR ABSORPTIVITIES OF THE DICHLOROTETRAAQUOCHROMIUM(III) IONIN PERCHLORIC ACID^

(8) J. W. Mellor, “ A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. X I , Longmans, Green and Co., London, 1931,p 386. (9) R . I>. Pecsok and J. J. Lingane, THISJ O U R N A L , 72, 189 (1950).

(10) Ch. Jorgensen, Acta Chem. Scnnd., 8 , 175 (1954) (11) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” Reinhold Publ. Corp., New York, N . Y., 1950, p. 601.

Wave length, mr

0.025 M“

aM for different chromium concn. 0.037 M“ 0.049 M a

0.04 M a

2.7 350 0.6 2.2 2.2 3.1 2.2 380 11.95 8.67 8.50 8.4 400 23.2 18.35 18.2 18.2 420 28.2 26.8 27.0 26.9 440 27.90 30.4 27.80 27.85 450 25.85 29.4 26.1 26.30 460 22.1 18.25 18.50 18.25 480 12.9 9.17 9.45 9.30 500 4.87 4.81 4.07 520 4.32 4.48 4.32 540 7.19 7.17 7.02 560 11.79 11.65 11.90 11.81 580 17.92 17.35 18.10 18.20 600 21.6 22.65 22.7 22.8+ 620 22.7 23.70 23.8 23.65 630 22.5 23.55 23.4+ 23.22 640 21.9 22.6 22.40 22.8 650 20.7 21.1 21.2 20.8 660 18.35 18.25 18.25 18.4 670 16.2 15.95 15.75 15.90 680 13.52 13.45 13.60 690 12.15 11.81 12.03 700 9.1 9.1 9.3 710 5.1 4.8 4.9 730 2.4 2.4 2.5 750 Measured in 6 M “ 2 1 0 4 solution. * Measured in 8.7 M HClO, solution. Detymined with a Cary Model 11 spectrophotometer a t 23

.

perchloric acid concentration; the relative absorbance of the second band increases with increasing acid concentration. When the solvent composition is changed from 6.0 to S.7 M in perchloric acid, the ratio of the absorbance a t 450 mp to that a t 635 mp is changed from 1.18 to 1.32 (Table 11). These effects indicate the probable formation of other species. The replacement of water by other ligands in the complex Cr(II1) ions may show two different kinds of spectral dependence: a bathochromic shift as in the chloroaquo complexes, or an alteration in the molar absorptivity as in thiocyanato complexes6b and the “o1” compounds.l0 The mean activity coefficient of 6.0 M perchloric acid is 4.76, and that of 9.0 M acid is 19.1.” Under these conditions, the extraordinarily high hydrogen ion activity may cause a “loosening” of the layer of water dipoles oriented about the Cr(II1) complex ion, which would permit the formation of outer sphere complexes with other ions in solution. It is noteworthy that the spectrum of hexaaquochrornium(111) perchlorate solutions is independent of perchloric acid concentration, whereas the spectrum of the dichlorotetraaquochromium(II1) chloride changes with the acid concentration (Table 11). This effect may well be due to the formation of ion pairs between the chloride ions in solution and the

1284

PHILIPJ. ELVINC AND BERNARD ZEMEL

\rol. 79

Cr(II1) complex, the tendency for complex formation being considerably less in the case of perchlorate ions than in that for chloride ions. While a mechanism of the type indicated in the previous paragraph is certainly speculative, it does present a possible explanation for the formation of [Cr(H20)3C13] under two very divergent sets of circumstances.

It has been shown that, for symmetrical hexacoordinated complexes such as the [Cr(Hz0)6]+ 3 ion, the longer wave length band of the two transition bands which appear, should be relatively simple, but that in the unsymmetrical ions, this band may be split into components. The nature of the spectra reported in the present investigation substantiates this. Formation of [Cr(H20)3C13].-TrichlorotriaquoFor compounds of the type [Cr(H20)4C12]+, chroinium(III) may be prepared by the solution of the short wave length component of the split band anhydrous dichlorotetraaquochromium(II1) chlo- for cis compounds has been shown to be ca. twice ride in anhydrous ether.7b The dichloro complex as intense as the long wave length component.12b disproportionates to give a solution of the brownish- The spectrum of the [Cr(H20)4C12]fion has a red trichloro complex; a transiently greenish- maximum a t 633 m p with an a M of 23.9 and a yellow solution is formed if water is added. It has shoulder a t 690 mp (ah* = 13.5). Since this band been demonstrated in the present study that, in the represents the superposition of two gaussian curves presence of concentrated (12 M ) hydrochloric acid, in which the intensity contributions of the two are the dichloro complex also converts to the trichloro superimposed, the ratio of the two intensities may spccies, forming a greenish-yellow solution ; on qualitatively be considered to be in the neighbordilution with water, the solution color rapidly hood of two so that by this criterion the species inchanges back to the green of the dichloro complex. volved appears to be the cis rather than the trum Since this reaction does not appear to take place form. in 5.G 111 calcium chloride solution, chloride ion by Experimental itself is not sufficient to drive the reaction forward. Apparatus.--A Cary Model 11 spectrophotometer, kept The trichloro complex may thus be formed from in a constant-temperature room maintained at 23 f 0.5", was used for recording absorbancies greater than one. The the dichloro ion either by the complete elimination Beckman model DU spectrophotometer used was equipped of solvent water in a non-aqueous medium or by with Beckman No. 2180 double thermospacers through which the presence in aqueous solution of high hydrogen water was circulated from a constant-temperature bath maintained at 25 =t 0.1'. Stoppered quartz 1-cm. cells ion and chloride ion concentrations. used. The spectrophotometric sample was balanced From the equivalence between high hydrogen were against a solution of the same electrolyte composition as ion concentration and the removal of solvent water, that used in sample preparation, except for the presence of one may postulate that the high hydrogen ion con- chromium species. Photometric Nomenclature.-The Bouguer-Beer law was centration functions to decrease the interaction where A = in the form of A = nMbC = -log (1/10), between the solvent water and the Cr(II1) com- used absorbance, U I I = molar absorptivity, C = concentration plex in the manner indicated. If the change in the (moles/liter), and b = path length (cm.). Preparation and Purification of Compounds. 1. Dispectral profile of the solution of the dichloro ion in chromic concentrated perchloric acid is indeed due to outer chlorotetraaquochromium(II1) chloride.-Green chloride is available (Merck) as a reagent grade salt; the sphere complex formation color and the ratio of free to complexed chloride indicate it to be largely dichlorotetraaquochromium( 111) chloride. There is, however, some variation with different samples, so that in order to have a consistent starting material, the (1) salt was purified. iZ saturated solution of the salt in 3.0 Af hydrochloric it appears reasonable to assume that the ion pair acid, maintained at 0" with an ice-salt mixture, was satformed may then react further in the presence of urated with hydrogen chloride. After precipitation was sufficient chloride ion with the outer chloride ion complete, the green crystals were filtered with suction and then dried in a current of clean dry air. The dry salt was exchanging with a complexed water molecule mashed well with acetone and then with anhydrous ether, the ether removed by suction, and the salt stored in a C1[Cr(H20)3C13] HzO (2) vacuum desiccator over Dehydrite. The final product is pale green anhydrous salt, [Cr(H20)rC!z]C1. thus providing a mechanism for the formation of theSolutions of the purified salt gave consistently reproduc[Cr(HzO)aCla] in concentrated hydrochloric acid. ible spectra which showed none of the rapid initial changes Configuration of [Cr(HzO)dC12]+.--The green observed in the unpurified material. Since there is only a clichlorotetra:tquochromium(III) ion may exist as small change in the percentage of complexed chloride ion, the probably is due t o the elimination of polynuclear either the cis or trans form. There is a t present no difference and/or other types of complexes from the unpurified salt. experimental verification of either form. Some 2. Chloropentaaquochromium( 111) Chloride.-This salt efforts have been made to determine complex ion was prepared initially by Weinland's classical method .I4 A saturated solution of dichlorotetraaquochromium( 111) chloconfiguration from absorption spectra. Tsuchida was aged at room temperature for 24 hr. A slight exattempted to correlate the position of a "third" ride cess of 70% sulfuric acid was added, and the precipitated band with cis-trans configuration; the results did [Cr(1~20)jC1]SO~ filtered. The precipitate was dissolved not, however? stand the test of time. * 2 in a iniuirnum amount of water, five volumes of ether added, The clevclopment of the crystal field theory has and the chloride prec-ipitat ed h y passing in dry hydrogen at 0". been applied with some success to the spectra of chloride I n the latter stages of the present investigation, an alterthe transition rleinent complexes in s o l u t i o ~ i . ~ native ~ ~ ~ 'method ~ was devised which seeins particularly convenient for the preparation of solutions containing known (11') ( a ) I